There are instances when it is appropriate to apply certain markings with regard to pharmaceutical moldings or similar moldings of foods, such as dextrosis or artifical sweeteners in tablet form. Examples of such markings include a bisecting strip, a warning note, an identification code, and a symbol related to intended use, such as a bed for sleeping tablets or a fruit for vitamin tablets. However, the application of desired markings is difficult due to the small size of moldings to be marked as well as the often non-planar surface of such moldings, a problem which also frequently causes difficulties when normal printing procedures, such as, for example, the roller rotation method, are employed.
Methods of contact printing of pharmaceutical moldings are described in several United States patents (see, U.S. Pat. Nos. 4,069,753, 3,272,118, 3,103,165, 3,889,591, 3,910,183, and 4,104,966). Common to all these methods are the problems arising from the facts that the prints smear off easily and that the printing cannot be effected with the same speed at which modern tablet pressing machinery runs. This latter fact means that contact printing is to be done as a special process, no matter what methods are employed.
It has therefore been conventional with regard to tablets to effect lettering or coding by means of engravings in the press tools during the pressing operation itself. It is necessary, in so doing, to provide for each preparation a special pressing tool, for example, a punch, provided with appropriate engravings.
This increases the cost of the manufacture of the moldings to a large extent, especially since, for example, an engraved punch is substantially more expensive than a nonengraved punch. In contrast thereto, an ink-jet-station is so variable that all necessary changes, for example, with regard to the codes, can quickly be arranged. In addition, such an embossing as mentioned above is not easily legible in certain light conditions and, in this respect, makes special demands upon an observer, as well as upon the quality of the granulate used.
Ink-jet systems are well known in the field of paper printing. In U.S. Pat. No. 4,029,006, a method of non-contact printing of cables is described, and in U.S. Pat. No. 4,168,662 an ink for the non-contact printing of food products, e.g., of waxed fruits, eggs, hard candy, and the like, is described, the ink in each case being quick-drying and smear-resistant. In all these cases the surfaces of the printed materials show either an equal hydrophilic or an equal hydrophobic character. The inks used for the printing of such materials can be easily adjusted to the qualities of the surfaces of these materials.
In contrast thereto uncoated tablets or uncoated tablet cores that is, compressed, solid, granulated masses of material, and even every single tablet or core, show varying characteristics with regard to their surfaces as a whole as well as to different zones, or areas, of their surfaces. Dependent upon the qualities of the active ingredients and the additional materials used to build up the granulate to be pressed, the surface of each uncoated tablet or uncoated core shows zones being more hydrophobic and other zones being more hydrophilic. In addition, dependent upon the thickness of the granulates to be pressed, the surface of the uncoated tablet or uncoated core can be either more or less smooth and plain or rough. This means that such a surface shows zones of different absorption which will influence the means in which the ink fluid is dispersed on the surface. Aside from these factors tablets of the same kind show zones of different smoothness and different absorption in the same species due to local differences in pressure exerted on the granulates by the tablet pressing tools.
Tablets usually show a decreased porosity in their fringe zones. The pharmaceutical tablets are, however, to be printed legibly even when they show a diameter of only a few millimeters. With all these facts in mind, as mentioned before, the realization of non-contact printing of tablets and cores with imprints of high quality standards using the ink-jet technique seemed to be out of reach.